The present invention relates generally to microturbine power generating systems. More specifically, the present invention relates to a tie-bolt for a microturbine power generating system.
The United States Electric Power Research Institute (EPRI), which is the uniform research facility for domestic electric utilities, predicts that up to 40% of all new electrical power generation could be provided by distributed generators by the year 2006. In many parts of the world lacking an electric infrastructure (e.g., transmission and distribution lines), the commercialization of distributed generators will be greatly expedited since central plants will not only cost more per kilowatt, but will also need expensive infrastructure installed to deliver power to the consumer. In the United States and other countries already having the electric infrastructure, the small, multi-fuel, modular distributed microturbine generation units will allow consumers of electricity to choose the correct method of electric service. The small, multi-fuel, modular distributed microturbine generation units will also allow consumers of electricity to choose the most cost-effective electric service.
Small, multi-fuel, modular distributed microturbine generation units could help alleviate current afternoon "brownouts" and "blackouts" that are prevalent in many parts of the world. U.S. Pat. No. 4,754,607, which is assigned to the assignee of the present invention, discloses a microturbine power generating system suitable for co-generation applications. A simple, single moving part concept would allow for low technical skill maintenance. Low overall cost would allow for wide spread purchase in those parts of the world where capital is sparse.
A microturbine power generating system includes a turbine stage, a compressor stage and an electrical generator, with each stage including a rotating component (e.g., a turbine wheel, a compressor wheel and a permanent magnet rotor). Supported by foil bearings, the rotating components are clamped together to rotate as a unit. A tie bolt is used for clamping the components together.
Temperature gradients in the tie bolt can lead to problems. During operation of the microturbine power generating system, heat propagates through the tie-bolt from the turbine end towards the compressor. A temperature gradient arises because the turbine operates at a much higher temperature than the compressor. Resulting from the heat gradient is thermal expansion that causes shifting of the rotating components and the loss of the tight interface between the rotating components. The shift causes the components to become unbalanced. The unbalance damages the bearings and eventually causes the bearings to seize.
Thermal stress in the tie-bolt can also lead to problems. Each time the microturbine power generating system is started, the tie-bolt undergoes a thermal cycle (cold-to-hot); and each time the microturbine power generating system is stopped, the tie-bolt undergoes a thermal cycle (hot-to-cold). The thermal stress could eventually lead to fatigue failure of the tie-bolt.
There is a need to reduce the chance of fatigue failure in the tie-bolt. There is also a need to reduce the thermal expansion in the tie-bolt.